This invention relates to a reproduction system for producing special-motion effects, such as slow motion, quick motion, still motion and other effects, in television video signals reproduced by a helical-scan video tape recorder player (VTR) with reproduction tape speed being different than recording tape speed, and more particularly to a digital time-base corrector for such a reproduction system.
Several systems have been proposed to record and/or reproduce television video signals on a magnetic tape. One system is the helical-scan video tape recorder (VTR) wherein a tape wrapped around a drum, including a rotary head, is transported to form video tracks or recorded paths of video signals on the tape diagonally with respect to the longitudinal direction of the tape. Such systems are widely used by consumers, industry, and by television professionals in broadcasting stations.
In the helical VTR, a video signal of one or more fields is usually recorded on a single video track, and the VTR rotary head is rotated in synchronism with a vertical synchronizing signal to accomplish recording of the input video signal. Hereinafter, each video track is assumed to contain one field of a television video signal. However the scope of the instant invention is not limited to such case as will be apparent from the following discussion.
In the case where the video signal of one field is recorded on one video track, the rotation of the rotary head is controlled so that the vertical blanking portion of the television video signal is recorded on one portions of each video track (the margin of the tape). In a single-head helical VTR wherein the television video signal is recorded by a single rotary head, the rotation of the single rotary head is controlled so as to rotate one revolution in one field period. In a dual-head helical VTR, wherein the television video signal is recorded by two rotary heads disposed apart from each other by 180.degree., the rotation of the rotary heads is controlled so as to rotate half revolution in one field period. Control of the rotary heads is accomplished by phase-comparing a tachometer signal, representing the rotating phase of the rotary head, and a vertical synchronizing signal for the television video signal to be recorded. Accordingly, the vertical synchronizing signal, for the television video signal to be recorded, maintains a predetermined phase relationship with the tachometer signal in the record mode.
For reproducing the recorded video signal, the rotary head is controlled so as to rotate at a predetermined rotational speed in synchronism with a synchronizing signal being supplied from a reference sync generator or other reference signals. In normal reproduction, wherein the tape is transported at normal tape speed, equal to the tape speed at which it was recorded, the running phase of the tape is controlled by controlling the rotation of the tape drive capstan. This is done so that the rotary head follows the center of the video track (achieves "tracking"), i.e., the head scanning path on the tape coincides with the video track.
A television video signal reproduced from a VTR contains time-base fluctuations due to mechanical fluctuation in the rotation of the rotary head and the tape transport speed. Such time-base fluctuation can be corrected by a digital time-base corrector (TBC). In a conventional digital TBC, as described in U.S. Pat. No. 3,909,839 entitled "PHASE LOCKING SYSTEM FOR TELEVISION SIGNALS USING DIGITAL MEMORY TECHNIQUES" issued to Inaba et al. and assigned to the present assignee, and in copending U.S. patent application Ser. No. 941,936 entitled "DIGITAL TIME-BASE CORRECTOR", issued July 22, 1980 as U.S. Pat. No. 4,214,262; by Mr. Mizukami, assignor to the present assignee, and one of the present joint inventors, a VTR-reproduced signal containing a time-base fluctuation is converted to a digital television video signal in response to a write-in clock pulse synchronized with a sync signal contained in the reproduced signal and the digital signal is stored in a digital memory. The stored digital television video signal is read out in response to a read-out clock pulse synchronized with a reference sync signal without time-base fluctuation, and is converted to an analog television video signal. Thus, a television video signal, containing a time-base fluctuation, is stored in response to a write-in clock pulse containing the fluctuation and is read out in response to a read-out pulse containing no fluctuation. The resultant read-out television video signal contains no time-base fluctuation.
Because the helical-scan VTR records the television video signal of one field on a single video track, the television signals recorded on two adjacent video tracks have very high correlation. Therefore, the helical-scan VTR, can produce a special-motion television video signal when the tape is transported at a reproducing tape speed, different from the normal tape speed, and the rotation of the rotary head is maintained at the recording rotational speed. The motion produced is proportional to the reproducing tape speed and, in this way, a special-motion reproduced television video signal, such as fast, slow and still-motion television video signals, can be generated.
Copending U.S. patent application Ser. No. 91,195 "VIDEO HEAD DEFLECTION APPARATUS FOR SPECIAL MOTION REPRODUCTION BY HELICAL SCAN VTR", issued Feb. 2, 1982 as U.S. Pat. No. 4,314,284, by Y. Sato and T. Konishi, assignors to the present assignee, of whom T. Konishi is one of the present joint inventors, proposes an improved special reproduction system wherein the rotary head is deflected in a direction vertical to the video track in response to reproducing tape velocity (tape speed and transporting direction). This method ensures that the head will follow a single video track during a single scanning even when the tape is transported at a speed different than normal speed. The rotary head is jumped by an amount equal to at least one video-track pitch in a vertical blanking period, as shown in FIGS. 4(1) through 4(10). The amount of additional head deflection (jump) is responsive to the reproducing tape velocity. In other words, in special-motion reproduction, the rotary head repeatedly scans the same video track (in still- or slow-motion reproduction) or skip-scans the video track (in the quick-motion reproduction). In special reproduction, before the rotary head completes scanning of a single video track, it is displaced to the position of another video track, thus causing a change in the number of horizontal scanning lines being reproduced.
The NTSC color television video signal has a sequence of four unique fields (4-field sequence) and interlace scanning is utilized. Thirty pictures are transmitted per second and a frame representing one picture consists of two fields, odd- and even-numbered, differing by one half of one horizontal scanning line with respect to the scanning position. The phase of the horizontal synchronizing signal for the odd field differs from that for the even field by half (1/2H) of one horizontal scanning period (1H). Further, the color sub-carrier frequency is determined to be 455/2 times the horizontal scanning frequency, and the number of horizontal scanning lines in one frame is 525. Therefore, the color sub-carrier phase of the first frame is reverse that of the second frame. This means that the video tracks, on which each NTSC color television video signal of one field is recorded, also include a 4 field sequence. In normal reproduction, therefore, the VTR reproduces a normal NTSC color television video signal having a 4 field sequence.
In special-motion reproduction, however, a VTR-reproduced television video signal never has a 4-field sequence because the rotary head performs repeat scanning (in still- and slow-motion reproduction) or skip scanning (in quick-motion reproduction). In still-motion reproduction, only the same television video signal of one field is repeatedly reproduced. In forward slow-motion reproduction, with a tape speed of 1/5 normal tape speed, the rotary head repeatedly scans a video track five times and then scans the following video track, whereby the same-field video signals are sequentially reproduced five fields by five fields. In forward quick-motion reproduction, with a tape speed twice as fast as normal speed, the rotary head scans every other video track, i.e., performs skip scanning, whereby the television video signal from odd, or even-numbered fields only is reproduced.
The VTR-reproduced television video signal is supplied to a digital TBC to compensate for time base error. In the digital TBC, the reproduced television video signal is digitized and written into a digital memory in response to a clock pulse train synchronized with the color sub-carrier of the reproduced television video signal. Information is read-out from the digital memory in response to timing of a normal NTSC color television signal having a 4-field sequence. In the case where repeat scanning is performed, in special-motion reproduction, the video signal written into the digital memory does not satisfy the interlace system. In contrast, read-out is achieved in interlace fashion. Although it looks like interlacing is performed on the read out video signal, in reality the same signal is repeatedly read out, and correspond to different places on the picture separated from the other field by 1/2H. This gives a phase-shift to the read-out signal in that the vertical synchronizing signal is phase-shifted by 1/2H. This further causes an oblique straight line on a recorded picture to be displayed in a stepwise or zigzag pattern on the read-out picture.
In the case where the same video signal of one field is repeatedly written into digital memory, the read-out sub-carrier phase is alternately in phase and out of phase with the write-in sub-carrier phase every frame. In other words, the phase difference between the write-in and read-out sub-carrier phases is changed every frame. This will cause another phase-shift in that the horizontal synchronizing signal is phase-shifted by one half (about 140 nanoseconds) of one cycle period of the color sub-carrier. These phase-shifts in the read-out signal cause visual shifts in the vertical or horizontal direction on the picture tube.